U.S. patent number 4,722,594 [Application Number 06/797,920] was granted by the patent office on 1988-02-02 for two-dimensional optical information processing apparatus.
This patent grant is currently assigned to STC plc. Invention is credited to Matthew F. Bone, Neil Collings, William A. Crossland, Anthony B. Davey.
United States Patent |
4,722,594 |
Crossland , et al. |
February 2, 1988 |
Two-dimensional optical information processing apparatus
Abstract
Bistable operation of ferroelectric liquid crystal smectic I* or
smectic F* cells is disclosed which uses a greater liquid crystal
layer thickness than is achievable with smectic C* material while
yet retaining bistability of operation. One or more such cells are
employed in two-dimensional information processing apparatus.
Inventors: |
Crossland; William A. (Harlow,
GB2), Davey; Anthony B. (Bishops Stortford,
GB2), Collings; Neil (Harlow, GB2), Bone;
Matthew F. (Bishops Stortford, GB2) |
Assignee: |
STC plc (GB2)
|
Family
ID: |
10569737 |
Appl.
No.: |
06/797,920 |
Filed: |
November 14, 1985 |
Foreign Application Priority Data
|
|
|
|
|
Nov 14, 1984 [GB] |
|
|
8428811 |
|
Current U.S.
Class: |
349/17;
349/172 |
Current CPC
Class: |
G02F
1/141 (20130101) |
Current International
Class: |
G02F
1/13 (20060101); G02F 1/141 (20060101); G02F
001/13 (); G03H 001/16 () |
Field of
Search: |
;350/35S,340,348,162.13,162.14,349,332 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Kondo et al, "A Practical Method of Preparing Thin Homogenous
Ferroelectric Smectic Cells for Electro-Optical Microsecond
Switches, " Japanese Journal of Applied Physics, vol. 22, No. 2,
Feb., 1983, pp. L85-L87. .
Submicrosecond Bistable Electro-Optic Switching in Liquid Crystals'
by N. A. Clark and S. T. Lagerwall. Appl. Phys. Lett., vol. 36, No.
11, pp. 889-901, Jun. 1980. .
`Smectic Liquid Crystal Textures and Structures` by G. W. Gray
& J. W. Goodby, (publ. Leonard Hill, 1984) p. 153. .
`Ferroelectric Liquid Crystal Electo-Optics Using the Surface
Stabilised Structure` by N. A. Clark, S. T. Lagerwall & M. N.
Handschy, Mol. Cryst. Liq. Cryst, 1983, vol. 92, pp. 213-234. .
`Soliton Switch in Chiral Smectic Liquid crystals` by P. E. Cladis,
H. R. Brand and P. L. Finn, Physical Review A, vol. 28, No. 1, pp.
512-514 (Jul. 1983). .
`Synthesis, Properties and Applications of Ferroelectric Smectic
Liquid Crystals, ` by J. W. Goodby, Ferroelectrics, 1983, vol. 49,
pp. 275-284. .
`The Phase Behavior of
bis-(4'-n-Heptyloxybenzylidene)-1,4-Phenylenediamine (HEPTOBTPD),
Crystal J and K Phases`, by P. A. C. Gane et al., Mol. Cryst. Liq.
Cryst., 1983, vol. 100, pp. 67-74..
|
Primary Examiner: Corbin; John K.
Assistant Examiner: Lerner; Martin
Attorney, Agent or Firm: Kerkam, Stowell, Kondracki &
Clarke
Claims
We claim:
1. Two-dimensional optical information processing apparatus
incorporating at least one ferroelectric liquid crystal cell
exhibiting bistability of operation, which cell has a smectic I* or
F* phase liquid crystal layer confined between opposed electroded
plates whose inward facing surfaces have been treated to promote
planar alignment of the adjacent liquid crystal molecules in the
same direction at each of the two surfaces, which plates serve to
define a liquid crystal layer with a thickness in the range from 4
to 40 microns.
2. Apparatus as claimed in claim 1, wherein the or each liquid
crystal layer thickness is less than 20 microns.
3. Apparatus as claimed in claim 1, wherein a pleochroic dye is
dispersed in the or each liquid crystal layer.
4. Apparatus as claimed in claim 1, wherein the or each cell is a
transmission type cell.
5. Apparatus as claimed in claim 1, wherein the or each cell is a
reflex type cell.
6. Apparatus as claimed in claim 1, wherein the or each cell is
adapted to operate in an amplitude modulation mode.
7. Apparatus as claimed in claim 1, wherein the or each cell is
adapted to operate in a phase modulation mode.
8. Apparatus as claimed in claim 1, which apparatus is a joint
transform correlator.
9. Apparatus as claimed in claim 1, which apparatus is a frequency
plane correlator.
Description
BACKGROUND TO THE INVENTION
This invention relates to two-dimensional optical information
processing (OIP) apparatus, and in particular to such apparatus
incorporating a ferroelectric smectic liquid crystal cell.
The first types of liquid crystal display cell to be used as
display devices employed nematic or cholesteric phases. Those that
operated in field effect mode could typically be operated with
signal strengths of a few volts, but when the exciting field was
removed, the liquid crystal always relaxed back into the same state
within a short period of time. Then our UK Patent Specification No.
1557199 described how an electrically addressable non-volatile
liquid crystal display can be provided. This employed a smectic A
liquid crystal filling to provide bistability of operation. A
drawback of this approach was that it required a significantly
greater drive voltage. Later, in a paper entitled `Submicrosecond
bistable electro-optic switching in liquid crystals` by N.A. Clark
and S.T. Lagerwall appearing in Applied Physics Letters Vol. 36 No.
11 pp 889-901 (June, 1980), a bistable cell switchable with smaller
voltages was described that employed a ferroelectric smectic C
liquid crystal filling.
The terms `bistable` and `bistability` are used here and elsewhere
in this specification in relation to a situation in which a liquid
crystal is electrically switchable between two latching states that
are optically distinct on a macroscopic scale so that under
appropriate illumination conditions, for instance by direct viewing
or by viewing in position between appropriately oriented crossed
polarisers, the cell is capable of functioning as a display element
electrically switchable between two latched conditions of
contrasting appearance.
In order to exhibit ferroelectricity, a smectic material must not
only exist in an tilted smectic state such as Smectic C, I or F,
but it must also be constituted by a material that is intrinsically
chiral, or it must include a chiral constituent to provide
chirality. For a definition of the ordering of the different
smectic phases, reference may be made to the book entitled `Smectic
Liquid Crystals Textures and Structures` by G.W. Gray and J.W.
Goodby, published by Leonard Hill (1984), and in particular to the
diagrams appearing on page 153 of that book. According to
convention chirality may be signified by an *, and thus the
material employed in the Clark and Lagerwall cell may be described
as a C* material.
The chirality of a ferroelectric liquid crystal material in a C*,
I* or F* phase means that its molecules have a natural tendency to
align themselves in progressively different directions in
succeeding smectic layers. If the layers are arranged in parallel
planes this progression defines a helix, and the pitch of this
helix is typically in the region of 2 to 3 microns unless it has
been lengthened by diluting the chiral molecules with non-chiral
ones or with further chiral molecules of the opposite
handedness.
The Clark and Lagerwall paper previously referred to describes the
bistable operation of a cell with a 1.5 micron thick layer of
DOBAMBC or HOBACPC maintained in a C* phase with its smectic layers
aligned in parallel planes perpendicular to the plane of the liquid
crystal layer itself. Under these conditions it was observed that
the tendency to helical arrangement of the liquid crystal molecules
had been suppressed, and the authors attributed the bistable
operation they found to this suppression of the helical structure
by surface stabilisation.
In a later paper entitled `Ferroelectric Liquid Crystal
Electro-Optics Using the Surface Stabilised Structure` appearing in
Mol. Cryst. Liq. Cryst. 1983 Vol. 94 pp 213-234 these authors, in
collaboration with M.A. Handschy, report further about work
undertaken with such cells, describes the bistability observed in a
cell containing a 2 micron thick layer of C* phase DOBAMBC, and in
both C* and I* HOBACPC in a cell 1.5 microns thick cell. (In this
paper the I* phase of HOBACPC has been incorrectly identified as
the F* phase, but this has been corrected in later
publications.)
The validity of the theory that the suppression of helix formation
is effective in providing bistability of operation has been
investigated by ourselves and others. In our work with C* phase
material we have been unable to demonstrate any bistability of
operation in cells providing a liquid crystal layer thickness of 4
microns or greater even when the pitch of the bulk material filling
the cell was several times greater than the layer thickness. This
finding is given support in the literature. Thus P.E. Cladis and
H.R. Brand, in a paper entitled `"Soliton switch" in chiral smectic
liquid crystals`, appearing in Physical Review A Vol. 28 No. 1 pp
512-4 (July 1983), report that in their investigations performed
using 10 microns thick cells filled with C* phase materials having
pitches in the range from 10 to 100 microns they found no evidence
of bistability even with switching fields as high as
6.times.10.sup.5 V cm.sup.-1. This paper concludes with the
sentence, `Furthermore, we have demonstrated that, in general,
production of samples with a thickness smaller than the pitch does
not lead to bistability in chiral smectics`. Similarly in a paper
entitled `Synthesis, Properties and Applications of Ferroelectric
Smectic Liquid Crystals` appearing in Ferroelectrics, 1983, Vol. 49
pp 275 to 284, in the section entitled `Applications`, J.W. Goodby
states that, `The smectic phases C*, I* and F* can be used in three
different ways. (1) Thin cells, 1-3 microns thick in which the
helix is unwound and the cell is bistable. The switching speed from
one tilt domain to another is in the microsecond range. Optical
contrast is achieved with crossed polarisers. (2) Thick cells where
the helix is unwound. The cell is not bistable but the switching
speed can still be in the microsecond range. This cell has similar
viewing angle properties and contrast to a conventional twisted
nematic device. (3) Thick cells where the helix is not unwound. The
cell is not bistable and has similar properties to (2)'.
In summary, for C* phase material of any pitch, no bistability of
operation has been reported for cells having a liquid crystal layer
thickness of greater than 3 microns. Similarly no prior art reports
any other type of ferroelectric cell with a liquid crystal layer
thickness greater than 3 microns that exhibits bistability of
operation. It is believed that, for pitches up at least to several
tens of microns, the pitch is unwound when the layer thickness is
less than the pitch. It therefore appears that the suppression of
pitch is not the paramount factor determining whether or not
bistability of operation is exhibited. We believe that bistability
is determined by the type of order present in the phase of the
material present in the cell.
SUMMARY OF THE INVENTION
The present invention is concerned with ferroelectric liquid
crystal OIP cells having liquid crystal layer thicknesses
significantly in excess of 3 microns that do exhibit bistability.
More particularly the present invention is concerned with the
discovery that the behaviour of I* and F* materials is sufficiently
different from that of C* materials to allow the essentially
bistable operation of I* and F* material filled display cells
having a liquid crystal layer thickness significantly greater than
the limit in the region of 3 microns that is characteristic of
known display cells employing C* material. This bistability in I*
and F* material filled cells does not necessarily require the
surface stabilised suppression of the helices by virtue of the
thinness of the liquid crystal layer. In the case of I* or F*
material it is believed that the unwinding of the helix is a bulk
stabilised effect resulting from an extensive three dimensional
bond orientational ordering present in such I* and F* materials.
The presence of a bulk stabilised effect, rather than a surface
stabilised one, may be directly inferred from the observation of
the behaviour of pitch lines in a cell whose liquid crystal layer
thickness is so great that pitch lines appear or are retained when
the cell is cooled into the I* or F* phase in the absence of an
applied electric field. When an electric field of sufficient
strength is first applied these pitch lines disappear, but when the
field is removed they do not immediately reappear. The presence of
the pitch lines, before the application of the field, indicates
that the layer was not surface stabilised; whereas the absence of
pitch lines after removal of the electrical field indicates that
stabilisation has been effected by some phenomenen other than
surface stabilisation. The surface thus appears to play a secondary
role in the bistable operation of these I* and F* cells.
Experiments have however revealed that this bistability does not
extend indefinitely with liquid crystal layer thickness, but has
been shown that at least with certain materials to exist out to
thicknesses well in excess of 20 microns.
According to the present invention there is provided
two-dimensional optical information processing apparatus
incorporating at least one ferroelectric liquid crystal cell
exhibiting bistability of operation, which cell has a smectic I* or
F* phase liquid crystal layer confined between opposed electroded
plates whose inward facing surfaces have been treated to promote
planar alignment of the adjacent liquid crystal molecules in the
same direction at each of the two surfaces, which plates serve to
define a liquid crystal layer in the range from 4 to 40
microns.
Generally, it is preferred to employ a liquid crystal layer
thickness of less than 20 microns.
BRIEF DESCRIPTION OF THE DRAWINGS
In the following description of preferred embodiments of the
invention, reference is made to the accompanying drawings in
which
FIG. 1 depicts a schematic perspective view of an OIP liquid
crystal cell,
FIG. 2 is a graph of a log log plot of switching pulse duration and
switching pulse voltage, for a specific filling and thickness of
cell constructed according to FIG. 1,
FIG. 3 is a similar graph plotting results obtained with a
different filling and thickness.
FIG. 4 is a schematic diagram of the electrode layout of a one
dimensional linear array cell.
FIG. 5 is a schematic diagram of a joint transform correlator
employing a pair of I* and F* ferroelectric liquid crystal cells,
and
FIG. 6 is a schematic diagram of a frequency plane correlator
employing a single I* or F* ferroelectric cell.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
A hermetically sealed envelope for liquid crystal layer is formed
by securing together two glass sheets 11 and 12 with a perimeter
seal 13. The inward facing surfaces of the two sheets carry
transparent electrode layers 14 and 15 of indium tin oxide, and
each of these electrode layers is covered within the display area
defined by the perimeter seal with a polymer layer, such as
polyimide (not shown), provided for molecular alignment purposes.
Both polyimide layers are rubbed in a single direction so that when
a liquid crystal is brought into contact with them they will tend
to promote planar alignment of the liquid crystal molecules in the
direction of the rubbing. The cell is assembled with the rubbing
directions aligned parallel with each other. The thickness of the
liquid crystal layer contained within the resulting envelope is
determined by the thickness of the perimeter seal, and control over
the precision of this may be provided by a light scattering of
short lengths of glass fibre (not shown) of uniform diameter
distributed through the material of the perimeter seal.
Conveniently the cell is filled by applying a vacuum to an aperture
(not shown) through one of the glass sheets in one corner of the
area enclosed by the perimeter seal so as to cause the liquid
crystal medium to enter the cell by way of another aperture (not
shown) located in the diagonally opposite corner. (Subsequent to
the filling operation the two apertures are sealed.) The filling
operation is carried out with the filling material heated into its
isotropic phase so as to reduce its viscosity to a suitably low
value. It will be noted that the basic construction of the cell is
similar to that of for instance a conventional twisted nematic,
except of course for the parallel alignment of the rubbing
directions. For one example the liquid crystal filling of a 10.7
micron thickness cell was the chiral ester ##STR1## this material
being marketed by BDH under the designation CE8. When heating this
material from the crystalline state the following transition
temperatures were noted:
______________________________________ Smectic J* to Smectic I*
67.degree. C. Smectic I* to Smectic C* 70.3.degree. C. Smectic C*
to Smectic A 80.7.degree. C. Smectic A to Cholesteric 135.degree.
C. and Cholesteric to Isotropic 140.degree. C.
______________________________________
When, after filling, the cell was slowly cooled the planar
alignment inducing nature of the rubbed polyimide surfaces caused
the alignment of the smectic layers formed on entering the smectic
A phase to be in planes normal to the rubbing direction, and hence
perpendicular to the planes of the major surfaces of the liquid
crystal layer. This alignment of the smectic layers was maintained
as the liquid cooled into the smectic C* phase even though the
orientations of the molecules within those smectic layers changed
upon entering the C* phase. At this stage pitch lines could be
observed. This indicated that the cell was too thick for unwinding
the helix solely by surface effects. This was to be expected since
the liquid crystal layer thickness was considerably greater than
the helix pitch (about 2 microns for this material in the C*
phase). A slight change of scale in the pitch lines (c. 10%) was
observed when, upon further cooling, the filling entered the I*
phase.
With the cell maintained at a temperature of 69.8.degree. C. so
that its filling was maintained in the I* phase, a series of tests
was performed to see how switching was affected by voltage. For
this purpose the cell was mounted between crossed polarisers
aligned with their polarisation planes at 45.degree. to the rubbing
direction. A plot of the results is illustrated in the graph of
FIG. 2 where the log of pulse duration is plotted as a function of
the log of pulse voltage. In practice the results were obtained by
increasing the voltage of a pulse of known duration. For each pulse
length there is plotted a transient effect threshold voltage at
which the effect of the switching voltage first becomes noticeable
but is not sustained. The corresponding bistable effect threshold
voltage is a measure of the larger voltage at which switching is
established with no significant relaxation even over periods
exceeding 2 hours. (It is believed that relaxation may not be
noticeable for a considerably longer period, but with the
particular apparatus being employed it was not possible to
establish this because adequate temperature stabilisation could not
be ensured over periods significantly longer than 2 hours.) From
FIG. 2 it is seen that the bistable switching time varies
approximately with the inverse of the voltage over the range 115
volts down to about 25 volts. At lower voltages down to about 12.5
volts it varies approximately as the inverse sixth power of the
voltage, while at still lower voltages, with pulse duration in
excess of 10 msec, it varies approximately as the inverse square of
voltage. When the cell was first bistably switched with an electric
field the helix lines disappeared and did not reappear while the
cell was maintained in the smectic I* phase. A second cell was
constructed using a thicker perimeter seal providing a liquid
crystal layer thickness of about 20 microns. This showed similar
properties of bistability to those of the 10.7 micron cell, though
the switching voltages were higher.
A third cell was then constructed with a spacing of about 30
microns. In this instance there was not true bistability, and
noticeable relaxation effects could be observed within a few
seconds of pulsing. It is believed that this results because the
liquid crystal layer is now so thick that the alignment of the
smectic layers in planes at right angles to the confining surfaces
is no longer adequately enforced throughout the layer thickness. It
is similarly believed that the extinction of bistability which is
observed in the material when in the smectic C* phase that is
observed when the cell thickness is expanded beyond a limit in the
region of 3 microns is attributable to the same cause, the smaller
thickness limit necessary for securing planar alignment of the
smectic layers being occasioned by the absence of the hexatic
structure of the material when in the C* phase.
It is to be clearly understood that the specific smectic I*
material exemplified is not held out to be an optimised material
for OIP applications. By suitable admixture of other materials the
temperature range of the I* phase can be shifted and extended.
Furthermore, the switching efficiency can be improved by the
incorporation of material with a higher polarisation than that of
CE8. By way of example a fourth cell with a 40 microns liquid
crystal layer thickness filled with non-chiral (racemic) CE8 to
which had been added approximately 6 wt % of a chiral ester
providing increased polarisation, and also a longer pitch, is still
not too thick to exhibit bistability of operation. Such additives
must leave the mixture in the I* or F* phase at the working
temperature, but do not individually have to exhibit either of
those phases.
By way of further example tests were also carried out on a cell
containing the alternative ferroelectric hexatic smectic phase,
smectic F*. For this purpose material known to exhibit an F phase
(from a paper by P.A.C. Gane et al appearing in Mol. Cryst. Liq.
Cryst. 1983, Vol. 100, pp 67 to 74) was rendered ferroelectric by
the admixture of a chiral ester dopant. The resulting mixture
comprised 20.8 wt % HEPTOBPD
(bis-4'-n-heptyloxybenzylidene)-1,4-phenylenediamine), 79.2 wt %
TBBA (terephthalylidene-bis-n-butyl aniline), and 6.1 wt % chiral
ester dopant. This mixture exhibits an F* phase from 110.degree. C.
to 130.degree. C. which is depressed by about 9.degree. from that
of the undoped HEPTOBPD/TBBA mixture. FIG. 3 depicts on a log log
scale how the switching pulse duration varied as a function of
switching voltage for an 8.9 microns thick layer of this F* phase
material when maintained at a temperature of 124.degree. C. The
dilution of the chiral dopant by the HEPTOBPD/TBBA mixture provided
a pitch helix for the bulk material significantly longer (c. 40
microns) than the layer thickness, and hence no pitch lines were
seen in this cell.
The ability to construct relatively fast switching bistable cells
with a liquid crystal layer thickness significantly greater than 3
microns is advantageous because it means that much of the
technology previously developed for other types of cell, such as
twisted nematic and phase change cells, can now be expected to be
applied with little modification to the manufacture of these cells;
whereas rather more significant changes would likely to be required
if the thickness range developed for those other types of cell had
to be substantially reduced to bring the thickness beneath the 3
microns threshold.
Attention is now turned to illustrative two-dimensional OIP
applications for cells of the general type described above. One
form in which such cells may be used for two-dimensional OIP is for
the creation of a two-dimension array of pixels. For a transmission
type multiplexed (matrix addressed) cell, the cell construction can
be substantially as previously described with reference to FIG. 1.
Transparent electrode layer 14 consists of a finely spaced set of
rows or columns, while electrode layer 15 consists of a similar set
extending in the orthogonal direction. Typically, with a view to
obtaining the maximum information density, the electrode rows and
columns are made as narrow as conveniently possible, and similarly
the inter-row and inter-column electrode spacings are also made as
narrow as possible. If the satisfying of these criteria means that
the inter-pixel spacings are not significantly smaller than the
widths of the pixels themselves, then it will generally be
preferred to arrange for the inter-row and inter-column electrode
spacings to be occupied by opaque material so that the modulated
light, that is the light that passes through the pixels, is not
swamped by the unmodulated light that would otherwise be able to
pass through the areas between neighbouring pixels.
Remembering that a ferroelectric cell is sensitive to the direction
of an applied electric field, many types of matrix addressing
systems developed for use with other types of liquid crystal mode
devices will not be suitable for this application. One type of
addressing scheme which does take account of this polarisation
direction sensitivity, and which is thus suitable for these cells
is based on the use of bipolar pulses, and is described in the
United Kingdom Patent Specification No. GB-A 2146473 to which
attention is directed.
For a reflex type multiplexed (matrix addressed) cell, in which a
specular reflector is located behind the liquid crystal layer, the
cell construction can incorporate an active silicon single crystal
slice to replace the transparent rear sheet of a transmission type
device. Such a sheet carries an array of metal pads whose members
are connected with the drains of an associated array of FET's
formed in the silicon, this array of FET's being accessed on a
co-ordinate basis. The pixels of the cell are defined by the
regions of the liquid crystal layer that lie between these pads and
a transparent counter-electrode carried on the inward-facing
surface of the transparent front sheet of the cell. Specular
reflection at the rear of the liquid crystal layer is required over
the areas registering with the pixels, and in some circumstances
this may be provided by the reflective properties of the metal
pads, but if the flatness is not adequate, then the whole surface
may be coated with dielectric material to improve the flatness, and
then a multilayer interference filter type reflector is deposited
on top of that. Since the device works in reflective mode, the
front sheet of the cell needs to be made of good optical quality
glass and should be provided with an anti-reflection coating on its
front surface.
Linear arrays of pixels, rather than two dimensional arrays, can
also be used for two-dimensional OIP, for instance by having two
linear arrays with one arranged side by side in the input plane
and, in the Fourier transform plane, some means for recording the
interference patterns of their Fourier transforms. A linear array
can have a construction similar to the two-dimensional array, with
the difference that, instead of having a set of column electrodes
facing the row electrodes, the row electrodes are faced by a single
large area counter-electrode. When particularly fine resolution is
required the problem may arise of accessing the individual
electrodes. One way of easing this problem is to employ a
two-dimensional accessing for a one-dimensional pixel array. An
example of such a system is illustrated in FIG. 4. The sheet on one
side of the liquid crystal layer carries a set of substantially
rectangular transparent electrodes 40 which extend widthwise across
the cell and are provided with terminal pads 41 and connecting
links 42. The sheet on the other side of the liquid crystal layer
carries a set of serpentine transparent electrodes 43 with terminal
pads 44. In this instance four electrodes 43 are depicted. All the
members of each loop register with each one of the rectangles 40 to
define overlapping areas, the pixels, having the form of a regular
array of uniformly spaced straight strips. The additional areas of
overlap, where the connecting links 42 are overlapped by the
serpentine electrodes 43, can conveniently be masked out with
opaque matter if they prove a nuisance.
The binary electro-optic effect provided by these cells, whether
they incorporate linear or two-dimensional pixel arrays, can be
employed in an amplitude modulation mode, a bipolar mode, or a
guest dyed amplitude modulation mode.
In amplitude modulation mode operation a transmission type cell is
irradiated with an extended wavefront of linearly polarised
coherent monochromatic light. The cell is arranged in relation to
this polarisation plane so that in one of the cell states the
liquid crystal director lies in that plane. The light transmitted
by the cell then passes through a polarisation analyser, which is
set with its polarisation plane in crossed relationship with the
plane of polarisation of the light incident upon the cell. The
birefringence and thickness of the liquid crystal layer within the
cell are chosen so that it will function as a half wave retardation
plate.
The effect of energising of the cell is to rotate the fast axis of
this half wave plate through the angle between the two director
states of the cell. Preferably, by choice of composition of the
cell, this angle is chosen to be near 45.degree. so that, with
appropriate orientation of the cell, energisation produces a
90.degree. rotation of polarisation plane. It may be noted that in
the I* and F* phases the angle between the director states is much
less heavily dependent upon temperature than in the C* state, and
therefore temperature regulation is not so critical.
In the phase modulation mode a similar arrangement applies, but
with the difference that in this instance the plane of polarisation
of the incident light bisects the angle between the two directors.
Under these circumstances switching produces a 180.degree. change
of phase instead of a change of amplitude. A primary advantage of
the resulting phase coding is that it reduces the d.c. spot
amplitude in the Fourier plane by destructive interference.
These amplitude modulation and the phase modulation modes of
operation can readily be adapted for use with reflex type cells
instead of transmission type ones. This involves dispensing with
the polarisation analyser, and instead directing the incident light
on to the cell via a polarisation beam splitter.
The analyser is also dispensed with in the amplitude modulation
mode if a guest pleochroic dye is incorporated into the liquid
crystal to give a guest dyed amplitude modulation mode. If the cell
is of the reflex type, the beam splitter is not a polarisation beam
splitter, but is of conventional construction not intended to be
polarisation sensitive.
FIGS. 5 and 6 depict respectively schematic representations of a
joint transform correlator and of a frequency plane correlator
which employ ferroelectric liquid crystal cells. In the case of the
joint transform correlator of FIG. 5, plane polarised light from a
laser 50 is beam expanded at 51 and directed on to a pair of reflex
type I* or F* ferroelectric cells 52 via a polarisation beam
splitter 53. One of the cells 52 carries the object data for
correlation with the scene data carried by the other cell. A first
Fourier transform lens 54 collects the reflected light and directs
it to interfere in the Fourier transform plane where is located a
recording device 55. For real-time correlation this may be for
instance a bismuth silicon oxide detector. This recording device is
interrogated with light from a second laser 56. Diffracted light is
collected by a second Fourier transform lens 57 and directed to a
photodetector 58 positioned in the Fourier transform plane. The
resulting output from the detector is fed to a processing unit 59
for utilisation.
In the case of the frequency plane correlator of FIG. 6, plane
polarised light from a laser 60 is beam expanded at 61 and directed
on to a single reflex type I* or F* ferroelectric cell 62 via a
polarisation beam splitter 63. A first Fourier transform lens 64
collects the reflected light and directs it to a recording device
65 positioned in the Fourier transform plane where it is
interferred with a collimated reference beam 66. A second Fourier
transform lens 67 is positioned to collect light that has been
reflected by the cell 62 now programmed with alternative
information, this light being such as to act on the stored
information of the recording device to reconstruct the reference
beam if there is correlation between the alternative information
content of cell 62 and its original information content. This light
collected by lens 67 is directed to a photodetector positioned in
the correlation plane.
Minor modifications to the correlators of FIGS. 5 and 6 are of
course necessary to adapt them for use with transmission type cells
rather than reflex type ones.
* * * * *